What would be the signs of leaky valve seals?

The most common is a puff of blue smoke out of the exhaust right after you start it up which clears up after it's running a few seconds.

Laky valves will also show smoke when vacaum is very high. Have someone follow you, run it up to 4000 in second, cruise there for a minute or 2, and take your foot off the gas, watch for smoke.